Optical communication systems use digital modulation techniques to convert digital information (e.g., bits) into symbols carried by optical signals (e.g., light pulses) through an optical fiber. Optical signals are capable of high-speed data transfer and also high quality data transfer because optical signals are not distorted by electromagnetic fields. An optical communication system includes an optical transmitter (TX) and an optical receiver (RX). At the optical transmitter, a digital data signal may be modulated into a phase-modulated and amplitude-modulated signal. The modulated signal includes orthogonal in-phase (I) and quadrature (Q) components, which are exploited by digital modulation techniques, such as quadrature amplitude modulation (QAM), to maximize the spacing between modulated symbols in order to maximize robustness of the symbols against distortions. The modulated signal may be converted at the transmitter into an optical signal and transmitted to the receiver via an optical fiber. The optical receiver may receive and demodulate the optical signal to recover the digital data.